Specularly reflective collectors which have been designed to date have not been well optimized in terms of both energy collection efficiency and control of flare radiation. This is due partially to the fact that there are no commercially available computer programs capable of optimizing such designs. The designer must rely on conceptual designs that can only be analyzed on a computer. In addition, there has not been a strong emphasis placed on maximizing collection efficiency, rather; much of the past effort would appear to have been directed toward minimizing flare radiation. Flare radiation is defined as that portion of the stimulating radiation, reflected or scattered by the storage phosphor, which enters the collector and propagates along a path, such that it exits the collector and strikes the storage phosphor at a position which does not coincide with the position of the scanning beam. This errant radiation will stimulate blue photon emissions from this other location and thereby corrupt the signal which is being detected synchronously with the position of the scanning beam, as well as the signal which will be detected from this other location if it has not yet been scanned. The net effect of flare radiation is to corrupt the fidelity of the latent image by reducing the overall dynamic range, and in particular, the contrast ratios in regions of low exposure.
When considering specularly reflective collector designs, the designer should be aware of a few important guidelines. First, the number of reflections required to reach the detector must be minimized in order to achieve high collection efficiency. For example, an aluminum reflector will absorb 8% of the incident 390 nm emission with each reflection. Second, referring to FIG. 1, the declination angle, .alpha., of the detector 4, with respect to the surface normal 2 of the storage phosphor 1, should be minimized. Given that the emissions are Lambertian in nature, the radiated energy which strikes the detector directly, without undergoing any reflection, is proportional to the cosine of this declination angle (i.e., the projected area of the source). Third, the cross sectional angle, .omega., subtended by the detector 4 should be kept as large as possible to maximize collection of the radiated energy which can strike the detector directly. This can be achieved by keeping .beta., the angle that the detector's mirror 3 makes with respect to the line of sight, as close to zero as possible, thus maximizing the projected area of the detector as viewed from the phosphor. In addition, the cross sectional angle, .omega., can be increased by minimizing the distance between the detector and the phosphor. Fourth, the detector may reflect a large portion of the incident energy, hence, the collector should be designed to return as much of this energy as possible back to the detector, with a minimal number of reflections. In the case of a photomultiplier tube with a K.sub.2 CsSb photocathode, the blue radiation reflectivity has been shown to be approximately 22%. And lastly, the design must prevent or minimize flare radiation.
U.S. Pat. No. 4,742,225 discloses a reflective collector design with elliptical cross section possessing very good collection efficiency. In this design, the detector's declination angle is approximately 20 degrees, and the cross sectional angular subtense of the detector is approximately 26 degrees. A closed form solution of the flux incident on an array of detectors at this location and orientation shows that approximately 20% of the emitted radiation will strike the detector array directly. In addition, the emitted radiation which fails to strike the detector directly, will strike it after a single reflection. The design has the following disadvantages. First, fabrication of the elliptical reflector can be difficult or costly. Second, no attempt has been made to recycle the energy reflected by the detector; rather the back side of the entrance aperture is made into an absorber to minimize flare radiation. If this mirror were made of a specularly reflective material, the bulk of the energy reflected by the detector would require 2 to 3 reflections to reach the detector again. Third, flare radiation is not as well controlled as in other collector designs. (See the following U.S. patents which disclose systems having these, as well as other disadvantages: U.S. Pat. Nos. 3,663,083, issued May 16, 1972, inventors Friedman et al; 4,346,295, issued Aug. 24, 1982, inventors Tanaka et al; 4,736,102, issued Apr. 5, 1988, inventor Morrone; 4,775,791, issued Oct. 4, 1988, inventors Owen et al.)
U.S. Pat. No. 4,743,758 discloses three specularly reflective box designs. In these designs, the detector's declination angle varies from 49 to 53 degrees. In addition, the detector is located at such a distance from the phosphor that it subtends a small cross sectional angle of only 11 to 16 degrees. At such a location, much of the emitted radiation must undergo numerous reflections in order to reach the detector. A closed form solution of the flux incident on an array of detectors at this location and orientation shows that approximately 7% of the emitted radiation will strike the detectors directly. The chief advantages of the reflective box designs are that flare radiation is well controlled, and the mirrors are relatively inexpensive to manufacture. The chief drawback of these designs is a lower collection efficiency due to: the small angular subtense of the detector, the number of reflections required to reach the detector, and the inability to recycle the reflected energy.
U.S. Pat. Nos. 4,743,759, 5,105,079, 5,134,290, and 5,140,160 disclose various designs regarding the use of tapered roof mirrors to direct the stimulated emissions towards the detector as well as to direct the scattered stimulating radiation away from the phosphor. These designs employ a very large diameter detector to increase the angular subtense of the detector and to increase the angle of the taper. However, the location and orientation of the detector with regard to the position of the stimulated emissions force these designs to rely heavily upon multiple reflections. Collection efficiency suffers because many of the parameters affecting the angular subtense of the detector become self defeating in these designs. For instance, the detector's declination angle, .alpha., is approaching zero degrees just as the angle that the detector's mirror normal makes with respect to the line of sight, .beta., is approaching 90 degrees, and vice versa. Likewise, the distance between the detector and the emission source decreases as the angle that the detector's mirror normal makes with respect to the line of sight, .beta., is increasing. In addition, the vertically oriented mirror essentially doubles the size of the upper entrance aperture, thereby allowing twice as much energy to escape from the collector. This results in a great reduction of collection efficiency near the far ends of the collector where this aperture is closest to the phosphor. The chief advantages of the roof mirror configurations are the low cost associated with utilization of a single detector and plane reflectors. In addition, flare radiation is extremely small.
U.S. Pat. No. 4,849,632, issued Jul. 18, 1989, inventor Watanabe; U.S. Pat. Nos. 4,591,714, issued May 27, 1986, inventors Goto et al; and 4,591,715, issued May 27, 1986, inventor Goto disclose stimulable phosphor imaging systems in which emitted light is collected by a solid transparent light guide assembly and detected by multiple photomultiplier tubes. The light guides disclosed in these patents are expensive, difficult to manufacture and rely on multiple light reflections to the PMTs, thus reducing light collection efficiency.
Radiation collection efficiency is also a function of the material or substrate which is utilized to construct the reflective surfaces which reflect the radiation to the photodetector. That is, once the collector light path geometry is optimized (in other words, how the light rays bounce, or travel from the initial collection aperture to the photodetector), it is the specular reflectivity that determines the efficiency of the light collector system.
The higher the reflectivity of the light collector, the better the light collection at the PMT. More specifically, the specular collection efficiency of the mirrors wants to be high and the diffuse reflection low. In addition to performance, manufacturability and cost are very important.
Potential collector substrate/mirror options include:
______________________________________ Aluminized estar on difficult to manufacture rigid support Aluminum-machined expensive, high diffuse and buffed reflectivity Aluminum-diamond expensive turned Commercially low specular reflectivity available first surface mirror, glass and plastic Vacuum metallized (Luster-Coate) low specular reflectivity substrate Optically coated acrylic higher diffuse reflectance substrate than optically polished and coated glass ______________________________________
As exemplified in the following patents, although acrylic has been used in a variety of optical applications, it has not been utilized to produce a radiation collector, for phosphor emissions, which requires very high specular reflectivity and low diffuse reflectivity.
U.S. Pat. Nos. 4,259,370, issued Mar. 31, 1981, inventor Fisli; 4,101,365, issued Jul. 18, 1978, inventor Fisli; 4,564,286, issued Jan. 14, 1986, inventor Maiorano; 3,800,058, issued Mar. 26, 1974, inventors Bartok et al.; 4,541,415, issued Sep. 17, 1985, inventor Mori; 4,068,121, issued Jan. 10, 1978, inventors Bringhurst et al.; 4,842,824, issued Jun. 27, 1989, inventor Ono.
There is thus a problem in the prior art of providing a light collector in stimulable phosphor imaging systems which has high light collection efficiency and flare radiation attenuation and low diffuse reflection.